There are demands for a colorless function of ROADM (Reconfigurable Optical Add/Drop Multiplexer). The colorless function has a merit of transmitting an arbitrary wavelength signal to a network. In order to achieve the colorless function, it is preferable that a frequency (wavelength) can be converted. For example, it is possible to convert a modulated light into a modulated light having a difference frequency by inputting a modulated light ω1 and an unmodulated light ω2 into a non-linear crystal and generating a difference frequency (ω2−ω1) (for example, see Japanese Patent Application Publications No. 2005-173572 and No. 2000-171841 hereinafter referred to as Documents 1 and 2).